1. Field of the Invention
The present invention relates to a duplex printing apparatus, and more particularly to a duplex printing apparatus that is capable of printing on both sides of a sheet in a single step and has a stencil printing device and the like.
2. Description of the Related Art
Digital thermal stencil printing apparatus is known as a simple printing method. In this stencil printing apparatus, a thermal head on which a number of fine heat-generating elements are arranged in a main scanning direction is brought into contact with a thermal stencil master (“master,” hereinafter), and the master is conveyed in a sub-scanning direction (master conveyance direction) perpendicular to the main scanning direction, while electrifying the head-generating elements in a pulsating fashion so that the master is thermally melt-perforated in accordance with image information. This simplex master is then wrapped around an outer peripheral surface of a perforated cylindrical plate cylinder of a printing drum having the plate cylinder on the periphery thereof, whereupon a press roller or other pressing means is used to press the outer peripheral surface of the plate cylinder via a sheet of printing paper (simply referred to as “sheet” hereinafter) used as a medium to be printed. As a result, ink that is supplied to an inner periphery of the plate cylinder is transmitted through the perforated sections of the simplex master and transferred onto the sheet, whereby a printed image is obtained. Hereinafter, the printing drum is also simply referred to as “drum.”
In the above-described conventional stencil printing apparatus that performs stencil printing on only one side of a sheet, the printing speed was increased (referred to as “slow-up” hereinafter) from a low speed to a set printing speed (referred to as “set speed” hereinafter) while performing printing, when starting printing. For example, in the case where the printing speed was set to the fifth speed level (120 rpm), if waiting until the printing speed becomes the fifth set speed level, latency time has occurred. For this reason, in order to respond to a user request for reducing the total printing time, a slow-up operation was performed so that the printing speed is increased by one speed level every time when one sheet is printed.
In stencil printing, in addition to simplex printing in which printing is performed on only one side of a sheet, duplex printing in which printing is performed on both sides of a sheet is often performed recently for the purpose of reducing paper consumption, the amount of space required to store documents, and the like. Therefore, Japanese Unexamined Patent Application Publication No. 2003-200645 and Japanese Unexamined Patent Application Publication No. 2004-26373, for example, propose a new duplex printing apparatus for responding to a user request for performing duplex printing.
In the duplex printing apparatuses described in these publications as well, as with the above-described stencil printing apparatus for performing stencil printing on only one side of a sheet, printing needs to be performed by performing a slow-up operation on the printing speed until the printing speed increases from a low speed to a set speed when starting duplex printing, in order to respond to a user request of reducing the total printing time.
However, in these duplex printing apparatuses, the surface-engraved image (65A) is surface-printed, and then the reverse-engraved image (65B) is reverse-printed, as described with reference to FIG. 1 through FIG. 6 and the like. However, surface printing and reverse printing are not performed on the same sheet during a single rotation of the plate cylinder (12), but are alternately performed in each rotation. Specifically, only surface printing is performed on the first sheet at the first rotation of the plate cylinder (12) when starting duplex printing. Then, surface printing on the second sheet and reverse printing on the abovementioned first sheet turned by a rotation/turning effect of the press roller (13) are performed simultaneously and continuously at the second rotation of the plate cylinder (12). Surface printing and reverse printing are not performed on the same sheet during a single rotation of the plate cylinder (12).
Specifically, the duplex printing apparatus having the above-described configuration did not take into consideration the slow-up operation for increasing the printing speed to the set speed.